TP53 is a critical tumor suppressor gene capable of inducing cell cycle arrest, senescence, and apoptosis. Canonically, the primary negative regulator of the TP53 protein product p53, Mdm2, is considered to regulate p53 through two mechanisms; 1) through direct binding to the p53 transactivation domain, suppressing p53 activity, and 2) through functioning as an E3 ubiquitin ligase capable of ubiquitinating p53, targeting it for nuclear export and degradation. In addition to Mdm2, a homologous protein, MdmX also functions in p53 regulation, primarily through binding and blocking the p53 transactivation domain in a similar mechanism to Mdm2. Both Mdm2 and MdmX knockout mice are embryonically lethal, and rescued completely with concomitant deletion of p53, indicative of their critical role in p53 regulation. The development of an Mdm2C462A knock-in mouse model that maintains Mdm2-p53 binding, but disrupts Mdm2 E3 ligase activity, was found to result in embryonic lethality, rescued with simultaneous deletion of p53. Surprisingly, this study suggests that Mdm2-p53 binding alone is not sufficient for p53 regulation, and implicates the Mdm2 RING finger domain as critical in p53 regulation. Along with disrupting Mdm2 E3 ubiquitin activity, the mutation also disrupts Mdm2-MdmX heterodimerization. Because the Mdm2C462A mutation disrupts both functions of the RING finger domain, the E3 ubiquitin ligase activity and the MdmX binding, it cannot be deduced which of these changes is causing the observed misregulation of p53. Despite intensive study, much remains unknown about how Mdm2 and MdmX function in vivo to regulate p53. In vitro this binding has been demonstrated to amplify or rescue Mdm2 E3 ligase activity towards p53, but its role in vivo is not yet clear. Recent development of an Mdm2Y487A knock-in mouse, which maintains the ability to bind to MdmX and p53, but has disrupted E3 ubiquitin ligase activity has allowed for the separation of these two Mdm2 RING finger domain functions. Through utilizing this model, we hope to further elucidate the function of the Mdm2 RING finger domain in p53 regulation, as further understanding p53 regulation is critical in the development of effective therapeutics. PUBLIC HEALTH RELEVANCE: We have very recently generated mice with a single-amino acid residue substitution of tyrosine to alanine at the Mdm2-487 residue (Y487A). In vitro this substitution was characterized to result in an Mdm2 protein fully capable of binding to p53 and MdmX, but devoid of E3 ubiquitin ligase activity. To our surprise, despite lacking E3 activity the Mdm2Y487A mice are viable and are overtly phenotipically normal. Preliminary study has demonstrated lack of E3 function and retain of MdmX binding of the in vivo Mdm2Y487A mutant protein, indicating that the Mdm2-MdmX binding, but not the Mdm2 E3 ligase function, is critical for p53 control, at least during embryogenesis and early development. Characterization thus far of the Mdm2Y487A mutant mice has provided further insight into the role of the Mdm2 RING finger domain in p53 regulation. A better understanding of both the function and mechanism of the Mdm2 E3 ligase and the Mdm2-MdmX interaction in p53 regulation, a goal of this investigation, will help to identify novel targets for the treatment of cancer by enhancing the efficacy of p53-activating therapeutic agents.